it would be simpler if your modeler , modeled the road on to the terrain ,for eg. in 3dstudio max ,you could create the terrain with a displace modifier ,on a plane with sections in width and height to the amount of tiles needed , then you could export and import in to MED and convert to terrain , and convert your road separately .

you could also lightmap it etc..

you can also convert both as a single model but that would miss out on real terrain performance.

coding it would be difficult ,however if I am not mistaken tust has spline roads ?